Abstract - Relationships of Female Distributions and Vegetation to Mate Access and Mating Tactics in a Low Density Population of Pronghorns (Antilocaprea ameicana)

McDonald, Peter M. 1987. Relationships of Female Distributions and Vegetation to Mate Access and Mating Tactics in a Low Density Population of Pronghorns (Antilocaprea ameicana). MS Thesis. University of Southwestern Louisiana. 94 p.


The types of social organization and mating system within gregarious ungulate species - as with many animal groups - are inextricalby linked with the nature of their envirionment. This is borne out by several studies detailing both inter- and intraspecific social variation under differing environmental pressures (reviews in Estes 1974; Leuthold 1977; Owne-Smith 1977). Not only do interpopulatonal differences occur within the same species, but social systems may temporally shift within a population. An important environmental factor is the availability of critical resources. Both quantity and quality of food items can affect grouping tendencies and spatial dispersion of individuals (Jarman 1974). The extent to which females associate together and are distributed in their environment may contribute, in turn, to the phenotypic expression of male strategies adapted to maximizing encounters with potential breeding partners (Gosling 1986).

Authors within the last fifteen years have attempted to synthesize research on the social and mating systems of ungulates in relation to varying environmental constraints, usage of resources, and behavioral patterns. Estes (1974) reviewed the social organization of African bovids, classifying them as solitary or gregarious and territorial or non-territorial. He suggested that social organization became increasingly complex as individuals radiated out from forest origins into the varied niches of open habitats. There, they underwent morphological and social changes as adapations to increased exposure to predators and unpredictability of resource abundance and distribution. Leuthold (1977) categorized social organization of African ungulates by their use of and relation to space, and by the type and size of the group in which individuals are usually found. Owen-Smith (1977) summarized research on behavioral traits of ungulate mating systems. He proposed that territoriality is a "low benefit, low cost" strategy that would be evolutionarily favored if the adoption of an alternative strategy resulted in an exces of > 10% annual mortality, relative to territoriality.

In an earlier paper Jarman (1974) related how a male's strategy to monopolize mates may be influenced by the feeding 'sytles' of the species. Ungulate foraging strategies are influenced by morphology, and they range from highly selective browsers that remove an entire plant item and consume small quantities with each bite, through intermediate feeding styles, to grazers that ingest large quantities of plant part per bite. Because of the dispersed nature of browse (forbs, aerial foliage, woody plants) - scattered individual items or small, widely scattered patches - browsers tend to be loosely associated in small, mobile groups. As dispersion of plant items increases with removal, individuals within the group are forced farther apart and must range over larger areas. Grasses tend to be more evenly distributed in the environment than is browse; grazers can therefore associate in large, closely knit groups that may forage more slowly and over smaller areas. Any subsequent grazers moving through the area can retain similar grouping patterns, because the dispersion of food items remains relatively unchanged, even though individual food items may be smaller. Thus, the foraging and grouping patterns of individuals is seen to be closely related. In terms of mate acquisition, a male's probability of locating and acquiring breeding partners is dependent on his adaptive responses to the stability and mobility of female groups moving through their environment in response to available resources (Jarman 1974).

Jarman's (1974) ideas were expanded by Gosling (1986) to address the various mating 'decisions' face by male African antelopes. Gosling proposed that the primary influence on male mating strategies is the movement of females in relation to their food supply. By either 'following' females, or 'sitting-and-waiting' for them while defending the resources that they need (resource defense territoriality, Emlem and Oring 1977), a male may enhance his reproductive success relative to female distribution during the reproductive period. However, in order to secure mating rights, the male must have previously estbalished his competitive status with other males. For 'followers', this takes the form of an 'individual reference for dominance' that is recognized among other following males from previous encouters. 'Sit-and-wait' males, by familiarity with their area and resource value, may be willing to escalate contests with intruding males. They establish a 'resource reference for dominance' that intruders unfamiliar with the area can assess.

An important component to both Jarman's (1974) and Gosling's (1986) ideas is the tendency of females to associate in groups. By doing this, the probability increases for a male to gain access to a larger number of females, than if they foraged as widely-spaced individuals. Females should feed unhindered by competition for food items if there is selection to maximize nutrient intake for survival, zygote production, and raising offspring to independence (Gosling 1986). Therefore, just as forage quality and availability may place upper limits on the size of a feeding or social group, some factor must also be setting lower limits on group size (Jarman 1974). Probably the most widely accepted explanation for group living is its function as an anti-predator strategy (Hamilton 1971; Pulliman 1973; Alexander 1974; Treisman 1975). Additionally, individuals in groups may benefit by locating and exploiting patchy, unpredictable resources (Horn 1968), as well as by spending proportionally more of the activity budget in foraging (Caraco 1975; Caraco et al. 1980; Lipetz and Bekoff 1982; Underwood 1982; Alados 1985).

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